Common media and technology families include CD, DVD, and Blu-ray Disc. Standalone, non-computer, optical storage devices also exist, for example popular CD players, DVD players, and some DVD recorders, but those are not covered in this article.
Double layer (DL) media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but require the optics to change the laser's focus. Traditional single layer (SL) writable media are produced with a spiral groove molded in the protective polycarbonate layer (not in the data recording layer), to lead and synchronize the speed of recording head.
